A diagnostic method is already known from U.S. Pat. No. 4,794,790 in which the controllability of a tank venting valve and of an idling regulator is checked. The tank venting valve is arranged in a feedline which connects an intermediate container, which accommodates fuel vapors from a fuel tank, with the intake area of an internal combustion engine. The intermediate container usually contains an activated carbon filter which only permits a particular maximum degree of charge, that is it can only accommodate a maximum quantity of fuel in the form of fuel vapors.
It is therefore necessary to purge the filter regularly. This is usually done by supplying air to the intake area of an internal combustion engine via the activated carbon filter after operating the tank venting valve. Since, the higher the degree of charging of the activated carbon filter the more this additional quantity of air is enriched with fuel, supplying this air to the intake area leads to a falsification of the air/fuel ratio supplied to the internal combustion engine. This must then be compensated by a control loop, a so-called lambda control. Since the control process carried out by the lambda control is normally quite slow, methods were introduced which redetermine precontrol values for the fuel supply during the operation of the internal combustion engine, that is learn adaptively (U.S. Pat. No. 4,831,992). In this method, a distinction is made as to whether an existing tank venting valve is activated or not, the assumption being that the tank venting valve is opened or closed in dependence on the activation.
In carrying out the diagnostic method presented in U.S. Pat. No. 4,794,790, it is assumed that the additional quantity of air which can be supplied by the tank venting valve is enriched so little with fuel vapors that this additional air (Q.sub.TEV) is comparable to an additional air (Q.sub.LLR) supplied by the idle actuator. By selectively activating the tank venting valve and the idling control, which can be controlled by an idling regulator, the operability of the tank venting valve and of the idle actuator is concluded from the responses of the idling control and of the lambda control. For the sake of completeness, it should be mentioned that the operability of the associated control chains, which essentially consist of amplifier stages and electrical connecting lines following the activation logic, is also inferred. However, the activation chains will no longer be explicitly mentioned in the text which follows.